1. Field of the Invention
The present invention relates to an improved apparatus and process for manufacturing a substrate sheet for information recording mediums such as optical disks and optical cards by continuously transferring preformat patterns to the surface of a resin sheet.
2. Related Background Art
Processes for manufacturing substrates for high-density information recording mediums such as optical disks and optical cards are hitherto known to include processes such as injection molding, compression molding and extrusion. In the injection molding or compression molding, severe conditioning is required for the temperature of molten resin, pressure applied thereto, precision of molds, etc. in order to assure flatness and smoothness of substrates and to prevent occurrence of warpage and inclusion of bubbles in substrates. In addition, an apparatus with a large scale is required, resulting in a high production cost. In the injection molding, it is also difficult to produce substrates as thin as 0.4 mm or less and having a low double refraction or birefringence as in substrates for optical cards. Moreover, in the compression molding and injection molding, substrates are formed sheet-by-sheet end hence their productivity is poor.
As for the extrusion, substrates for information recording mediums can be manufactured by bringing e roll stamper fitted with a stamper member, into pressure contact with the surface of a molten-resin sheet extruded from an extruder and continuously transferring patterns corresponding to preformats such as tracking Grooves or prepits to the surface of the resin sheet. Hence, the extrusion is advantageous for the mass-productivity of substrates, and in recent years 00researc thereon has been put forward as a process for manufacturing substrates for information recording mediums.
Incidentally, roll stampers used in the extrusion, for stamping-molding substrates for information recording mediums are conventionally manufactured by the following methods.
(1) A method in which preformat patterns are directly formed on the corresponding periphery of a roll substrate whose periphery has been mirror-finished (hereinafter "specular roll substrate").
(2) A method in which a flexible stamper member with preformat patterns 23 previously formed on its surface as shown in FIG. 2 is stuck or fixed to the corresponding periphery of a specular roll substrate using an adhesive or by a mechanical means.
Of the above conventional techniques, the method (1) makes it necessary to change the whole roll stamper for a new one even when only one fault is produced in the preformat patterns, resulting in poor molding efficiency. Hence the method (2) is mostly used.
A process of forming a substrate sheet for information recording mediums by extrusion using a roll stamper 24 prepared by the above method (2) will be described below with reference to FIG. 3.
First, resin pellets fed in an extruder 31 is heat-melted in a barrel of the extruder 31, pressed forward by a screw, formed into a sheet by means of a sheet extrusion die 32, and is continuously extruded from the sheet extrusion die 32 as a resin sheet 33 that stands substantially in e molten state. The sheet extrusion die 32 is so provided that the resin sheet is extruded to the gap between a specular roll 34 and the roll stamper 24. Subsequently the molten-resin sheet 33 is pressed between the specular roll 34 and the roll stamper 24 so that the preformat patterns 23 are transferred to the surface of the molten-resin sheet. Thus a substrate sheet 35 for information recording mediums is formed. Here, when width W.sub.33 of the molten-resin sheet 33 is so set as to be larger than width W.sub.21 of the flexible stamper member 21 as shown in FIG. 4, a cross-sectional view along the line A--A' in FIG. 3, molding efficiency can be improved since the preformat patterns 23 can be arranged even in the vicinity of widthwise both end portions of the flexible stamper member 21. This is therefore a preferred embodiment.
However, the formation of the substrate sheet for information recording mediums by extrusion in this embodiment has often caused a problem as follows: The flexible stamper member 21 of the roll stamper 24 is fitted in a thickness of 300 to 500 .mu.m in the case of a stamper member made of glass or 100 to 200 .mu.m even in the case of a stamper member made of metal such as Ni so that its strength or the precision of preformat patterns can be ensured. Hence, as shown in FIG. 5, which is a cross section in the direction parallel to the axis of the roll stamper (hereinafter "width direction"), a difference in height h.sub.21 of at least 100 .mu.m is produced on the surface of the roll stamper. Thus, when the substrate sheet is stamping-molded using this roll stamper and also in the state of W.sub.33 &gt; W.sub.21 as stated above, a difference in gap between the roll stamper 24 and the specular roll 34 is produced at end portions of the roll stamper and at an area over which the flexible stamper member is provided, corresponding to t.sub.61, and t.sub.61, as shown in FIG. 6, a cross section along the line A--A' in FIG. 3, and hence the ends of the molten-resin sheet 33 toward the outsides of the widthwise both ends 63 of the flexible stamper member 21 (hereinafter "resin sheet edges") are not pressed between the roll stamper 24 and the specular rolls 34 and 36. As a result, the heat at the resin sheet edges can not be immediately removed to cause a great difference in cooling rate between the resin sheet edges and a preformat pattern transfer area, bringing about the problem that strain is produced at the preformat pattern transfer area.
Another problem has occurred such that, because of insufficient cooling at the resin sheet edges which tends to give the edges a self-supporting action, the resin sheet edges hang down to cause deformation in the preformat pattern transfer area of the resin sheet, or that the resin sheet edges hanging down and having solidified cause vibration of the roll stamper when they pass between the specular roll 34 and roll stamper 24 or between the roll stamper 24 and the specular roll 36, resulting in a poor transfer precision of preformat patterns.
These problems remarkably occur when the molten resin is made to have a high temperature. On the other hand, a method in which the temperature of the molten-resin sheet is enhanced is studied as a method by which the birefringence-free properties of substrate sheets or the transfer performance of preformat patterns are improved. Thus, the problem on the resin sheet edges is a very important technical subject in the manufacture of substrate sheet with a higher quality.